1. Field of the Invention
The present invention relates to an apparatus and method for producing a single crystal using the Czochralski technique.
2. Related Prior Art
Recently requirements for the quality of a semiconductor crystal substrate has increasingly become severer keeping pace with the trend of progressive increase in scale and precision of integration in a semiconductor device. Semiconductor crystals are mainly produced using the Czochralski technique or pulling technique and efforts on purification, a lower defect density and higher quality-wise homogenenity thereof have been continued. More recently, it has been found that not only purification of raw materials and parts used in a puller and improvement of the mechanical precision of the puller but also a history of thermal conditions during growth (heretofore also simply referred to as a thermal history) give a great influence on generation of defects in the bulk of a crystal. For instance, in the case of a silicon single crystal, it has been established that OSF (Oxidation Induced Stacking Fault), oxygen precipitation, BMD (Bulk Micro-Defect), FPD (Flow Pattern Defect), LSTD (Laser Scattering Tomography Defect), dielectric breakdown strength in oxides and the like are affected by a thermal history during growth. Besides, it has been made clear that, in the cases of such compound semiconductors as GaP, GaAs, InP and the like too, a dislocation density and defect levels acting as a donor or acceptor receive an influence of a thermal history during growth. In light of such accumulated knowledge, various pullers having different internal structures have been proposed for controlling of crystal defects through adjustment of a thermal history, one of which pullers is disclosed, for example, in an article of H. Yamagishi, I. Fusegawa, K. Takano, E. Iino, N. Fujimaki, T. Ohta and M. Sakurada: Proceedings of the 17th international symposium on Silicon Materials Science and Technology, SEMICONDUCTOR SILICON 1994, p 124.about.135.
However, these apparatuses and methods for operating the same are only capable of raising or lowering a temperature at a special position of an internal part in a puller, but not capable of adjustment of a whole temperature profile in the puller. Controlling a temperature at a special position of the internal part can be realized only through totally changing temperatures on a growing crystal with almost no freedom for profile adjustment and besides the precision of the controlling is poor. What's more, it is very difficult for them to control a special temperature range only. When a new temperature profile is required for the purpose to effect a task, each of these apparatus has to be basically changed in the design of the whole structure, which means that there is a disadvantage that each has to be newly designed into a fundamentally different apparatus.